1. Field of the Invention
The present invention relates to a semiconductor light-emitting device, such as a semiconductor laser or a light-emitting diode, which uses a compound consisting of elements in the second and sixth groups of the periodic table.
2. Description of the Related Art
Conventionally, various compound semiconductors have been used in semiconductor light-emitting devices such as a semiconductor laser. Recently, as a compound semiconductor for use in semiconductor light-emitting devices, a compound semiconductor (to be abbreviated as a II-VI compound semiconductor hereinafter) such as ZnSe has attracted attention. This semiconductor can emit visible light because of its wide bandgap (wide gap), which is equal to or larger than the energy corresponding to the wavelength of light in the visible wavelength region, and consists of elements in the second and sixth groups of the periodic table.
A semiconductor laser or a light-emitting diode (LED) made from a material using a compound semiconductor (to be abbreviated as a III-V compound semiconductor hereinafter), such as GaAlAs or InGaAlP, which consists of elements in the third and fifth groups of the periodic table operates at wavelengths longer than that of green light. In contrast, the operating wavelength range of a semiconductor laser or a light-emitting diode using a II-VI compound semiconductor is relatively short, i.e., up to the wavelength of blue light or ultraviolet light. For this reason, the use of a II-VI compound semiconductor makes it possible to realize a semiconductor light-emitting device which has the advantages of conventional semiconductor light-emitting devices, e.g., a small size, a light weight, a low operating voltage, and a high reliability, along with the ability to emit light in the short wavelength region. High-concentration optical disks and full-color outdoor message boards can be expected to be realized by the use of such a II-VI compound semiconductor device.
FIG. 1 is a schematic sectional view showing the structure of a conventional current injection type semiconductor laser which emits a bluish-green light (bluish-green semiconductor laser) using ZnSe based materials as the II-VI compound semiconductor alloy. Referring to FIG. 1, reference numeral 1 denotes an n-GaAs substrate. On this n-GaAs substrate 1, an n-ZnSe layer 3, an n-ZnSSe layer 4, an n-ZnSe layer 5, a CdZnSe quantum well layer 6, a p-ZnSe layer 7, a p-ZnSSe layer 8, and a p-ZnSe layer 9 are stacked in sequence via an n-GaAs buffer layer 2. A p-side Au electrode 11 is formed on the p-ZnSe layer 9 in the form of a trench type electrode which is isolated via a polyimide layer 10. An n-side In electrode 12 is formed on the other side of the n-GaAs substrate 1.
Applied Physics Letters, Vol. 59, pp. 1,272-1,274 (1991) reports that a bluish-green semiconductor laser with the above arrangement can operate under a continuous wave excitation at liquid nitrogen temperatures and under pulsed excitation up to room temperature.
The above bluish-green semiconductor laser structure, however, has not resulted in the continuous wave excitation at room temperature which is required for a practical semiconductor laser. This is because the use of a wide-gap II-VI compound semiconductor such as ZnSe not only raises the operating voltage significantly but also increases nonradiative recombination processes in the light-emitting layer, thereby decreasing luminous efficiency, as compared with that of a III-V compound semiconductor. In addition, when a semiconductor layer consisting of a wide-gap II-VI compound semiconductor is epitaxially grown on a semiconductor substrate consisting of a III-V compound semiconductor such as GaAs, defects are produced at the interface between the semiconductor substrate and the semiconductor layer, and this further decreases the luminous efficiency.